Feed has conventionally been improved in various ways in order to enhance the quality of eggs of poultry including chickens. For example, attempts have been made to improve quality and productivity of eggs including a survival rate of chickens, an egg-laying rate, the Haugh unit of eggs and eggshell strength by adding to feed e.g., vitamin B6 and substances having antioxidative effects such as tea extracts, polyphenol, isoflavon agricons and plum vinegars (See Patent Literatures 1-5).